A conventional set of golf clubs normally contains a putter, several irons and a few woods. The heads of woods were traditionally made of wood, but more recently have been constructed of steel. A wood made of steel (also called a metal wood) has a similar external configuration to a wooden one, and has a hollow interior chamber to maintain the mass of the metal wood to be similar to a wooden one of the same designation (e.g. 3 wood).
More recently, metal woods have been made of titanium and other lightweight, high strength alloys. The use of these alloys has permitted club head designers to strategically design the heads due to fewer weight limitations in the material used. Titanium has a higher strength-to-weight ratio than steel, and therefore the entire golf club head can be made, for example, much larger than a steel or wooden head of the same designation.
The primary advantage of increasing the size of the club head is the increased size of the striking face of the club head. The striking face is the nearly flat, outer surface of the club head which is intended to impact the golf ball. Although a larger striking face improves the probability of hitting a ball in the desired direction, further improvements can be made to metal woods, whether of normal size or not.
FIG. 1. shows a conventional golf club 10 and its club head 12 swung along a swing path 14. The center of mass (which is denoted CM, and is coincident with the center of gravity) is the point at which the mass of the club head 12 is treated as being concentrated, and the swing path 14 is the line through which the center of mass passes when the club 10 is swung.
It is preferred that, when the striking face 16 impacts the golf ball 18, the plane of the striking face 16 be laterally perpendicular to the swing path 14 if a straight drive (no hook or slice) is desired. Laterally perpendicular means that a line drawn horizontally through the plane of the face 16 is perpendicular to the swing path 14. It is also preferred for a straight drive that the face 16 stay laterally perpendicular through the entire time period that the ball 18 and face 16 are in contact. Normally, some angle will be formed between the face 16 and swing path 14, but it is desirable to minimize this angle. When the face 16 strikes the ball 18, it compresses the ball 18, storing up energy which is released when the ball 18 expands and leaves the face 16. The more the face 16 remains laterally perpendicular until after the ball 18 leaves it, the less sideways spin (which causes hooks and slices) will be imparted to the ball 18.
Even if the swing path 14 starts laterally perpendicular to the striking face 16, if the point of impact with the ball 18 is not positioned along the swing path 14, the ball 18 will most probably not be projected away from the striking face 16 along a laterally straight line. Problems arise when the point of ball 18 impact is offset from the swing path because this may cause the face 16 to become laterally angled relative to the swing path 14 even if it is laterally perpendicular at the instant of impact. The changing of the angle between the face 16 and the swing path 14 is described in the following paragraphs.
FIG. 2 shows the golf club 12 swung along the same swing path 14 as in FIG. 1. The ball 18, however, impacts the striking face 16 at a point which is offset from the swing path 14 by the distance a.sub.2. At the instant the club striking face 16 contacts the ball 18 it exerts a force against the ball 18, and the ball 18 exerts an equal and opposite force against the club head 12. Since the force exerted by the ball 18 is applied to the club head 12 a distance a.sub.2 from its center of mass, the force causes a torque .GAMMA. to be applied to the club head 12. The distance a.sub.2 is the moment arm of the torque .GAMMA. (torque, .GAMMA. equals the force, F times the moment arm, a.sub.2) applied to the club head 12. The torque .GAMMA. causes the club head 12 to rotate about the point of impact between the ball 18 and the striking face 16, making the club head 12 an unstable body.
The rotation of the club head 12 about the point of impact with the ball 18 causes the striking face 16 to become laterally angled relative to the swing path 14 during the brief time period of ball impact. From the moment of initial impact of the ball 18 with the striking face 16, through compression and then expansion of the ball 18 until release of the ball 18 from the striking face 16, the lateral angle of the striking face 16 relative to the swing path 14 will change due to the torque .GAMMA. applied to the club head 12. Even if the face 16 is laterally perpendicular to the swing path 14 at the beginning of ball 18 impact, the torque applied to the club head 12 will rotate the club head 12, forming an angle between the face 16 and the swing path 14. The angled face 16 will cause sideways spin on the ball 18 making the ball 18 spin and therefore veer to one side or the other.
Increasing the club head size does not decrease the above-described torque. Increasing the club head size primarily increases the striking face size, thereby increasing the likelihood of hitting a golf ball with the better part of the striking face. However, since the striking face is so much larger on a larger club head than a conventional golf club head, the torque due to offset ball impact is potentially greater since the point of impact with the ball can be offset substantially farther from the center of mass, creating a substantially longer moment arm.
Therefore, the need exists for an improved golf club head which reduces or eliminates the torque applied to a golf club head due to misalignment of the ball with the swing path.